The present invention relates to a method and apparatus for improving the efficiency of a combined cycle power plant using excess thermal energy from the gas turbine exhaust of a power plant to supply hot water (xe2x80x9cdistrict waterxe2x80x9d) to a hot water system or network in a more thermally efficient manner. In particular, the invention relates to a new method for converting excess thermal energy into electric power by integrating a modified Kalina-type thermodynamic cycle into a district heating and cooling system in an efficient and cost-effective manner. The principal benefit of the new method is the recovery of additional heat for use with district water, which in turn can be used to heat homes and buildings by conventional means (e.g., radiators or other standard heat transfer devices). The invention also relates to the use of a thermal energy source from a gas turbine exhaust in a more efficient manner during the summer months (when demand for district water heating is low) by incorporating an intermediate pressure turbine into the process in an alternative embodiment in accordance with invention.
In the past, district heating and cooling systems have been integrated into power plants to take advantage of excess thermal energy sources. However, virtually all such systems have been integrated into plants that rely on traditional Rankine bottoming cycles. A conventional Rankine cycle has a relatively low thermal efficiency. Thus, the use of additional Rankine cycles to recover waste heat, particularly in bottoming cycles, normally is not cost-justified because of the efficiency losses inherent in such designs.
The Kalina thermodynamic cycle has previously been utilized to increase the thermodynamic efficiency of natural gas turbine combined cycle applications such as the process described in U.S. Pat. No. 5,440,882 (known generally as a xe2x80x9cKalina 6xe2x80x9d cycle). The working fluid in a Kalina 6 cycle consists of a multi-component mixture that normally has only one low boiling point component and one high boiling point component. The preferred form of the working fluid is ammonia and water (NH3/H2), although other high and lower boiling point mixtures of different components are theoretically possible. The mixture increases the thermodynamic efficiency of the system by virtue of the non-isothermal vaporization and condensation characteristics of the multi-component fluid which allow for heat absorption and rejection but with significantly lower irreversibility. This known characteristic of Kalina cycles allows the mixture to recover heat at lower temperatures than pure water in Rankine cycles. A regenerative boiler is typically used in a Kalina cycle in order to increase the overall efficiency of the system by vaporizing part of the working fluid using the superheated vapor of the high pressure (xe2x80x9cHPxe2x80x9d) turbine outlet, thereby increasing the vapor production in a heat recovery vapor generator (xe2x80x9cHRVGxe2x80x9d).
A conventional Kalina cycle has not heretofore been utilized in direct combination with district water heating and cooling systems, particularly as defined by the present invention using a direct, single stage condenser, in general, Kalina cycles include complex distillation condensation subsystems (xe2x80x9cDCSSxe2x80x9d) and are more expensive to construct and operate as compared to a Rankine cycles because they require additional process equipment and more costly materials of construction for NH3/H2O applications.
The design in accordance with the present invention shares a common element with the conventional Kalina cycle, namely, the regenerative boiler. Nevertheless, a need remains in the art for a method to increase the thermal efficiency of the traditional Rankine cycle district water heating and cooling systems by taking advantage of the potential increases in thermal efficiency of a Kalina bottoming cycle to heat and cool district water. A need also exists to decrease the number of processing units (and overall installation and operating costs) associated with the use of a conventional Kalina cycle to perform such tasks.
The present invention achieves those objectives by integrating a thermally efficient Kalina-type bottoming cycle with district water heating and cooling loads in the manner described below. The invention also includes a regenerative economizer that functions in combination with the regenerative boiler for achieving additional thermodynamic gains.
The preferred method for implementing a simplified Kalina thermodynamic cycle with a district water heating and cooling plan in accordance with the invention includes the following basic process steps:
(1) vaporizing a working fluid by transferring thermal energy from a gas turbine (xe2x80x9cGTxe2x80x9d) exhaust to the working fluid in the HRVG;
(2) expanding the working fluid through the HP turbine to produce electric energy, as well as a shell-side heat source;
(3) transferring thermal energy from the HP turbine outlet to the working fluid through a regenerative economizer and a regenerative boiler to produce a superheated, pressurized vapor working fluid and spent exhaust stream; and
(4) condensing the working fluid by transferring thermal energy to the district water.
An alternative embodiment of the invention includes steps (1) and (2), but modifies the step of expanding the vaporized working fluid by passing the expanded fluid leaving the HP turbine through a second intermediate pressure (xe2x80x9cIPxe2x80x9d) turbine to produce additional power, condensing the working fluid through a condenser and recombining the condensed working fluid with the condensate from the district water unit.
In order to carry out the above method steps, the present invention also includes an apparatus for improving the thermal efficiency of a combined cycle plant that includes the following basic components:
a. a heat recovery vapor generator (HRVG) for vaporizing, superheating, and pressurizing a working fluid;
b. a high pressure turbine for expanding the working fluid to generate electric power;
c. an intermediate pressure turbine for expanding the working fluid to produce power;
d. a first heat exchanger for cooling the multi-component working fluid exhaust leaving the high pressure turbine, while vaporizing recycled working fluid, preferably with the high pressure exhaust stream on the shell side and the recycled fluid on the tube side; and
e. a second heat exchanger downstream and in series with the first exchanger for further cooling the multi-component working fluid exhaust and heating the recycled working fluid, preferably with the vapor exhaust on the shell side and the recycled on the tube side;
f. a condenser in series with the first and second heat exchangers for condensing the cooled working fluid on one side and heating district water on the side.
In the preferred treatment method, the working fluid is pressurized, vaporized and superheated in the HRVG. The low boiling temperature component in the working fluid is combined with the high boiling temperature component to allow the mixture to capture thermal energy at a lower temperature and, in effect, store more energy. The working fluid is then expanded through the HP turbine to produce electric power in a conventional manner. The exhaust from the HP turbine then serves as a heat source for the regenerative economizer and the regenerative boiler. Thus, in the preferred embodiment, virtually all of the sensible heat from the HP turbine outlet is transferred either to the district water in the condenser or to the working fluid in the regenerative economizer and regenerative boiler.
In another aspect of the invention, the HP turbine outlet stream can be used as the working fluid for the IP turbine to create additional electric power. Integrating the district water heating system with a modified Kalina cycle in accordance with the invention results in the Kalina cycle being simplified in form, thereby reducing unit construction and operating costs. As noted above, a conventional Kalina cycle has power generation as the primary objective and thus does not include a regenerative economizer or a regenerative reboiler for use with district water. Conventional Kalina cycles also typically include significantly more complex distillation/condensation subsystems downstream from the IP turbine. Normally, such systems do not directly condense the vapor created by the HRVG, but instead rely on a multistage DCSS and plant cooling water subsystem.
The modified Kalina cycle according to the invention includes a regenerative economizer that transfers thermal energy from the superheated vapor leaving the HP turbine to the working fluid after the working fluid passes through the economizing section of the HRVG. The regenerative economizer thereby increases the thermodynamic efficiency of the system over a Rankine cycle having a district water heating cycle by making use of the excess thermal energy in the HP turbine outlet. The residual energy in the HP turbine outlet stream is then transferred to the district water for heating.
If the primary objective of the system is district water heating (rather than supplemental power generation when the ambient air temperature is low), the working fluid vapor can be directly condensed with the district water. Under those circumstances, a majority of the thermal energy from the HP turbine outlet is transferred to the district water. As a result, the condensation subsystem downstream from the IP turbine can be significantly reduced in size, complexity and cost by using a single pre-heater and a single condenser.
Table 1 shows a comparison in the thermodynamic efficiency between a 3 pressure-level reheat Rankine cycle and a Kalina cycle as used in accordance with the invention and held at a constant district heating load (xcx9c280 MW). The design case reflected in the data of Table 1 involves the use of a 9 FA+gas turbine under nominal xe2x80x9cwinter-likexe2x80x9d conditions, i.e., 0xc2x0 C. and 60% relative humidity.